For image coding schemes undesirable ringing artifacts can appear in the decoded images.
Ringing artifacts tend to occur around edges in decoded images that are coded by coding schemes, such as by Joint Photographic Experts Group (JPEG) schemes and Motion Picture Experts Group (MPEG) 1/2/4 schemes. Generally, a sharp edge in the spatial domain spreads energy over nearly the entire spectrum in the frequency domain. In low bit rate Discrete Cosine. Transform Based (DCT-based) coding, however, the high frequency coefficients are either removed or quantized coarsely, resulting in Gibbs phenomena around edges in the spatial domain, which is manifested as ringing artifacts in the decoded images. Ringing artifacts are more complicated for coded image sequences because they are rendered in motion, due to a different ringing pattern for every sequential frame. In coded image sequences (e.g., motion video), the ringing artifacts are often referred to as a “mosquito” or “corona” artifacts.
Generally, a de-ringing technique removes ringing artifacts from images while maintaining the edges and details of images. Many techniques have been developed for de-ringing. These techniques typically fall into two categories: first those that perform processing within the decoding loop of the image decoder; and second those that perform processing after decoding the image.
The techniques in the first category use coding information available in the decoding loop of the image decoder. These techniques are usually based upon information associated with the coding blocks (e.g., DCT coefficients) defined by the coding scheme.
The techniques in the second category use post-processing that is independent of the decoder, and therefore can be implemented independent of the decoder. Without coding information, the post-processing techniques tend to process every pixel. Processing every pixel of the image has much more calculation load than using information contained within coding blocks.
Many post processing algorithms that are sufficiently computationally simple to be implemented in low-cost TV sets use a form of low-pass filter (LPF). Unlike traditional linear LPFs, these modified LPFs preserve sharp edges while reducing ringing artifacts. There are other sophisticated techniques such as those based on principles of maximum a posterior probability estimation (MAP) or projection onto convex sets (POCS), but these are generally not suitable for inexpensive hardware implementations.
Noise reducing techniques can be used for reducing ringing artifacts. These techniques normally do not take into account the characteristics of the ringing artifacts, so their success is somewhat limited.